1. Field of the Invention
The present invention relates to a transparent cover plate that shields a solid state imaging device of an image pickup device mounted in, for example, a digital still camera or digital video camera.
2. Description of the Related Art
Conventionally, in the art of digital still or video cameras, an optical low-pass filter or an infrared cut-off filter is interposed between a taking lens and an image pickup device to improve the quality of images taken by the cameras. The optical low-pass filter is used to suppress moiré fringes caused by beats or interference between a periodic pattern of an object and a pitch of pixels, in the image pickup device. The infrared cut-off filter prevents adverse effects of infrared rays to a captured image. However, the conventional structure for mounting the optical low-pass filter or infrared cut-off filter requires a comparatively large space between the taking lens and the image pickup device. Accordingly, conventional structures limit design flexibility along the optical axis between the taking lens and the image pickup device.
The foregoing problem may be settled by disposing the optical low-pass filter or infrared cut-off filter as close to the image pickup device as possible. However, when the optical low-pass filter or infrared cut-off filter is disposed closer to the image pickup device, an unfavorable effect to the quality of the taking images, caused by dust on the filters, increases. Further, the conventional optical low-pass filter, infrared cut-off filter and cover glass are insulators, thus they attract dust in the air by electrostatic attraction. Particularly, when lithium niobate (LiNbO3) is utilized as an optical low-pass filter, as disclosed in Japanese Unexamined Patent Publication No. 11-282047, the filter attracts a considerable amount of dust since lithium niobate has strong pyroelectricity characteristics.